A blue-blooded creature has roamed the Earth for 445 million years, making it one of the most valuable beings in the world.

The history of Earth spans over 4.5 billion years, during which it has become home to an astonishing number of species: some have long since gone extinct, while others continue to thrive today. For instance, the remarkable creature whose blood is deemed the most valuable in the world first appeared on the planet long before the first dinosaurs — around 445 million years ago, according to IFLScience.

This creature is the horseshoe crab — a fossil animal that roams along coastlines. This species has very peculiar blue blood, which scientists consider to be the most precious in the world. Researchers note that the unique property of horseshoe crab blood is crucial for the development of vaccines that have saved millions of lives.

Unlike the iron-rich blood that flows in our veins, the blood of Atlantic horseshoe crabs (Limulus polyphemus) is rich in copper, giving it an icy blue color. Instead of leukocytes, they possess cells known as amoebocytes — which are extremely effective at detecting bacterial endotoxins. Even at a concentration of less than one part per trillion, amoebocytes can induce coagulation, forming a clot around invading bacteria in this manner. Researchers have found that this is what protects the horseshoe crab's body from toxins.

Pharmaceutical companies have utilized Atlantic horseshoe crab cells for decades as a means of testing vaccines for bacterial contamination, known as the Limulus Amebocyte Lysate test. The effectiveness of this test has resulted in horseshoe crab blood reaching an astounding price — around $15,000 per liter. Furthermore, scientists believe that with nearly every vaccine we receive in our lives, we are likely indebted to the horseshoe crab.

It is important to note that this test is critical for medicine, as otherwise, people risk exposure to contamination that could cause harm or even death. Fortunately for the Atlantic horseshoe crab, researchers are searching for more sustainable alternatives, as overharvesting has led to a decline in their population.

It is known that hunters of Atlantic horseshoe crabs take about 30% of their blood and then return the animals to the ocean. However, studies show that between 10% to 30% of horseshoe crabs do not survive this blood collection. Even more concerning, scientists have found that females breed less successfully after blood collection — making it difficult for the species to recover.

Researchers note that blue blood is not the only unique feature of Atlantic horseshoe crabs. In fact, they are not crabs at all and do not belong to the class of crustaceans.

Earlier studies have already shown that this species is actually related to spiders, as horseshoe crabs are members of the primary subphylum of arthropods. This group also includes arachnids and scorpions — both of which must shed their exoskeletons as they grow — the same applies to horseshoe crabs. When the time for molting comes, horseshoe crabs crawl out from the front of their shell, leaving behind a small exoskeleton.